Aromatic carboxylic acids such as terephthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylic acid are useful monomers for the preparation of polymeric materials, particularly polyester materials. For example, 2,6-naphthalenedicarboxylic acid can be reacted with ethylene glycol to prepare a high performance polyester, poly(ethylene-2,6-naphthalate) (PEN). Fibers and film manufactured from PEN have improved strength and superior thermal properties relative to other polyester materials. Films made from PEN demonstrate, for example, superior resistance to gas diffusion and particularly to the diffusion of carbon dioxide, oxygen and water vapor. Because of its exceptional properties, PEN is especially suitable for applications such as food and beverage containers, particularly for so-called "hot-fill" food and beverage containers, tire cord, magnetic recording tape and electronic components.
Methods for preparing 2,6-naphthalenedicarboxylic acid include the bromine-promoted, metal-catalyzed, liquid phase oxidation of 2,6-dialkylnaphthalenes. Such processes are disclosed in U.S. Pat. Nos. 3,870,754; 4,950,786; 4,933,491 and 5,183,933. The bromine-promoted, metal-catalyzed, liquid phase oxidation of 2,6-dialkylnaphthalenes, particularly 2,6-dimethylnaphthalene, produces a crude product containing a variety of impurities such as brominated 2,6-naphthalenedicarboxylic acid, 2-formyl-6-naphthoic acid, 2-naphthoic acid, trimellitic acid and other impurities. These impurities, particularly 2-formyl-6-naphthoic acid, are difficult to remove from crude 2,6-naphthalenedicarboxylic acid. Typically, customers desire that the crude 2,6-naphthalenedicarboxylic acid be purified before it is polymerized to form polymeric materials.
Methods for the purification of 2,6-naphthalenedicarboxylic acid are known. In the aforementioned U.S. Pat. No. 4,933,491, for example, 2,6-naphthalenedicarboxylic acid was purified after reacting the 2,6-naphthalenedicarboxylic acid with a lower alkanoic anhydride to produce a component that is soluble in excess alkanoic anhydride. The "solubilized" 2,6-naphthalenedicarboxylic acid was optionally treated with one or more purification procedures. U.S. Pat. No. 3,649,680 to McNarney discloses a process for purifying aromatic carboxylic acids wherein a mixture of water and an alkanol are added to an impure carboxylic acid paste, the carboxylic acid is separated from the alkanol/water mixture, and the purified carboxylic acid is subsequently washed with water. U.S. Pat. No. 3,671,578 to Ogata discloses a process for preparing 2,6-naphthalenedicarboxylic acid wherein the monoalkali salt of 2,6-naphthalenedicarboxylic acid is heated in water or a water-containing organic solvent, causing disproportionation thereof into 2,6-naphthalenedicarboxylic acid and the dialkali salt of 2,6-naphthalenedicarboxylic acid. U.S. Pat. No. 3,888,921 to Yamamoto et al., discloses a process for purifying 2,6-naphthalenedicarboxylic acid wherein an aqueous solution of a dialkali salt of crude 2,6-naphthalenedicarboxylic acid is prepared, then 40 to 97 mole percent of the dialkali salt is precipitated as a monoalkali salt while maintaining the pH of the aqueous solution at a value of not lower than 6.3, and converting the precipitate to 2,6-naphthalenedicarboxylic acid. It is disclosed in the Yamamoto et al. patent that the aqueous solution of the dialkali salt of 2,6-naphthalenedicarboxylic acid can be at a temperature of 60.degree. C.-350.degree. C. in the presence of potassium or sodium hydroxide, and it is disclosed that the solution can be treated with a reducing agent such as hydrogen gas, sodium dithionite, lithium aluminum hydride or sodium borohydride. U.S. Pat. No. 3,781,346 to Norton discloses a process for purifying naphthalene carboxylic acids comprising reacting a solid ammonium salt of the acid with steam at a temperature of from about 200.degree. C. to about 300.degree. C. U.S. Pat. No. 4,794,195 to Hayashi et al. discloses that it is impossible to purify crude naphthalenedicarboxylic acid to a high purity only by crystallization, and that it is necessary to combine the method of crystallization with other methods such as thermal treatment, oxidative treatment or reductive treatment. However, no specific means for conducting such treatments on 2,6-naphthalenedicarboxylic or other naphthalenedicarboxylic acid is disclosed. USSR Inventor's Certificate No. 486,008, to Kulakov et al. published on Jan. 15, 1976, discloses a method for purifying 2,6-naphthalenedicarboxylic acid by treating impure 2,6-naphthalenedicarboxylic acid having a particle size of 0.05-0.35 mm and containing up to 30% naphthalenemonocarboxylic acid with an aliphatic carboxylic acid at 180-250.degree. C. U.S. Pat. No. 5,563,294 to Sikkenga et al., discloses a process for purifying 2,6-naphthalenedicarboxylic acid using hydrogen and a noble metal supported on carbon as a hydrogenation catalyst. U.S. Pat. No. 3,584,039 to Meyer discloses a process for preparing fiber-grade terephthalic acid by catalytic hydrogen treatment of dissolved impure terephthalic acid using a carbon catalyst containing noble metals. The processes described are either complicated or require the use of a potentially expensive noble metal catalyst. What is needed is a simpler process for purifying aromatic carboxylic acids, preferably one that does not require the use of potentially expensive metal catalysts.